Bicycle internal gear hubs are well-known and have been in production for over a century. Most of these hubs operate by means of a planetary gear train, composed of three elements: a central sun gear; a planet gear holder which positions several planet gears around the sun gear; and a ring gear externally surrounding these planet gears. Many gear ratios are available by simply preventing the rotation of one of these elements, and choosing the remaining two elements as either the input or output of the transmission.
Compared to the popular derailleur chain transmission, a bicycle planetary hub is more compact, less vulnerable to damage, and less maintenance intensive. Furthermore, it allows gear changes when the bicycle is stopped. However, there are several significant disadvantages to internal hub transmissions.
Firstly, the internal friction of a hub gear is considerably greater than that of a derailleur system. This is due to the meshing of many small gears, all in a compact assembly, causing considerable friction. Since transmission efficiency is generally lower at lower pedal forces (Wilson 2004), internal friction is of greater importance when a bicycle is ridden at moderate speed. Hub-geared bikes are commonly purchased for their in-town practicality, and therefore pedalled at moderate urban speeds. Therefore the higher internal friction of hub gears is a significant disadvantage for their intended market.
Secondly, hub gears cannot withstand as much torque as derailleur gears because the gear teeth are considerably smaller that the sprocket teeth of a derailleur system. This torque limitation can prevent hub gears from being used in tandem bicycles, where the applied torque is higher. In general, gears are better adapted to high speeds and low torques, and chains to low speeds and high torques. Since a bicycle is a low speed, high-torque machine, it is not surprising that chain transmissions, such as the derailleur, dominate the bicycle market.
Thirdly, it is not practical to convert current bicycle gear hubs to other drive configurations. For example, current hubs are designed for chain-drive and are incompatible with direct-drive. With direct-drive, the pedals drive the hub directly with no intermediate chain or shaft drive. Direct-drive, particularly in the front-wheel-drive recumbent format (FIG. 1), has distinct advantages. It gives a very simple bicycle with no chain, a 50/50 weight distribution, and good braking stability (Garnet 2009). Current hubs are incompatible with direct-drive for several reasons: a) the axle is not strong enough for direct pedal load torsion; b) the gear ratios are too low; c) the gear tooth strength is insufficient for the applied loads; and d) the configuration of the hub is incorrect for proper crank mounting. If a hub could be made with internal gearing adaptable to both chain-drive and direct-drive, the larger available market would make direct-drive bicycles cheaper and thus more cost effective. There are advantages for chain-drive bicycles also: since a 1:1 ratio chain-drive is needed, a smaller front sprocket could be used, giving a more compact chain loop (FIG. 2), and allowing a smaller and lighter chain case.
Fourthly, currently available hubs transmissions are generally incompatible with mono-blade mounting. The term “mono-blade mounting” means that the wheel is mounted on one side only and the front fork or rear stay therefore has only a single blade (FIG. 3). Mono-blade forks and stays have several advantages over the standard double-bladed forks and stays. With mono-blade mounting, the tire can be changed without removing the wheel. Mono-blades can also improve the aerodynamics of the bicycle, and simplify the design of the frame (Burrows 2000). The design of folding bikes is also simplified, giving a more compact fold. Mono-blade mounting can also be used with direct-drive (FIG. 4). Currently available hubs are incompatible with mono-blade mounting because there is insufficient rigidity in the axle for one-sided installation. A hub that is compatible with both mono-blade and conventional double-blade mounting would offer more options for the bicycle frame designer, and reduce the cost compared to specially built mono-blade hubs.
These deficiencies of current bicycle transmission hubs point to a need for a hub having reduced internal friction and increased torque capacity, and also to a need for a hub that is adaptable to emerging new bicycle designs.
With these needs in view, it is an object of the present invention to provide a bicycle hub transmission with reduced internal friction. It is a further object of the invention to increase the allowable torque which can be applied to the hub. Another object of the invention to provide a bicycle hub transmission that is readily adaptable between chain-drive and direct-drive. And finally, it is an object of the invention to provide a bicycle hub transmission that is readily adaptable between conventional and mono-blade mounting.